1. Field of the Invention
The present invention relates in general to digitally controlled microwave frequency signal sources and in particular to a signal source producing an output signal having a frequency that may vary over a wide range in a flexible and precisely controlled manner.
2. Description of Related Art
Microwave voltage controlled oscillators (VCO's) are the heart of many microwave devices. They are used in many communications devices and in microwave frequency test equipment. Like all oscillators, the VCO exhibits less than ideal characteristics including drift with time and temperature. Because the VCO is designed to change its frequency as a function of a control voltage, the VCO also exhibits another non-ideal characteristic, non-linearity of its frequency as a function of voltage. Even if the VCO did not have these problems, the small noise content present on any control voltage will FM modulate the VCO to a very undesirable degree. This noise FM modulation is in addition to the VCO's inherent random frequency modulation caused by random noise in the oscillator and the fact that all oscillators use a frequency determining network that has a limited Q. This random frequency modulation is called phase modulation. These factors are well known in oscillator literature.
When used in receivers, a microwave VCO is often the best alternative for providing the required LO signals. But the problems mentioned above cause a VCO to be a less than ideal component. To correct the oscillator's problems it is common to phase lock it to a frequency reference. One simple way of doing this is to sample the instantaneous amplitude of the oscillator's output signal at a submultiple of the oscillator's frequency and use the resulting error signal to control the VCO's frequency. A specially designed sampling circuit called a phase gate is used for this purpose. It is an electronic switch that periodically closes for a very short period (e.g. 50 pS) at a rate precisely controlled by a reference signal. According to well known theories, this switch will produce output signals that are the difference between the microwave VCO's frequency and the harmonics of the reference signal.
For example, the signal from a microwave VCO that is intended to operate at 2,000 MHz is sent to a sampling gate that is driven by a 2 MHz signal. The difference signal from the phase gate will be
Phase Gate Output Freq.=.vertline.VCO Freq.-N * Ref. Freq..vertline. where N is an integer. The 1000th harmonic of the reference signal is at 2,000 MHz. Therefore, if the VCO's signal is slightly too high, for example 2,000.5 MHz, the phase gate output frequency will be 0.5 MHz. The sample output frequency will be obtained if the VCO is at 1999.5 MHz. If this signal is passed through a suitable compensation amplifier to the control terminal of the VCO, the VCO will be tuned to exactly 2,000 MHz and held there. The advantage to this technique is that relatively simple hardware can transfer the frequency drift and phase noise characteristics of a low frequency oscillator to a microwave frequency VCO.
It is often desired to make the VCO tune or to linearly sweep over a wide frequency range. For instance, in a spectrum analyzer in wide spans a microwave VCO will be required to tune roughly an octave from about 2,000 MHz to 3,800 MHz to cover the spectrum analyzer's input frequency range. The analyzer's user may choose to examine an input frequency in detail that requires the VCO to exhibit low phase noise, very low frequency drift and high frequency accuracy at any frequency. This requires some arrangement to phase lock the microwave VCO at any frequency in its range.
This presents a problem. Note that if the microwave VCO is locked to the 1000th line of the reference and if the reference drifts by 1 Hz, the VCO drifts 1000 Hz. Similarly, if the reference oscillator has some phase noise, the VCO will have N times as much phase noise impressed upon it over the bandwidth of the phase lock loop. Therefore, the reference signal must be very pure. It is difficult to make a reference signal simultaneously tune over a large percentage of it output frequency and be very pure. Instead the reference signal is made to tune just far enough so that the VCO can be locked on the (Nth) harmonic of the reference signal at one tuning extreme and on the (Nth+1) harmonic at the other. This permits locking the microwave VCO at any point in its tuning range.
There is a problem if it is desired to tune or sweep the microwave VCO beyond the range where the reference signal can tune. In previous art, the tuning range of the reference signal was somewhat extended to allow some overlap of the microwave VCO's locked tuning range. To tune or sweep over a wider range, the system had to halt and spend time unlocking the microwave VCO, tuning the reference to the other end of its range, and re-locking the microwave VCO on the next reference harmonic. Obtaining a lock by the microwave VCO to the correct harmonic of the reference is often a complicated process which takes some time. Limitations in this process cause the selection of very high performance microwave VCO's that can be operated in certain span ranges without the benefit of a phase lock.
What is needed is a signal source which can produce a high frequency output signal having a frequency which can vary with time over a wide frequency range in an accurately controllable manner.